1. Field of the Invention:
The present invention relates to a cab suspension system for trucks and, more specifically, relates to a suspension providing minimum bending moment or twist to a truck cab rear sill.
2. Description of the Problem:
It is desirable to isolate vehicle cabs and their occupants from shock and vibration of a truck""s chassis. This provides for the occupants"" comfort and extends the life of the cab by reducing stress which promotes structural damage to the cab, particularly to spot welds used to attach thin sheet metal components used for cab components such as the floor and the vehicle""s skin to the cab""s frame.
The load carried by a truck is supported by a frame which is resiliently carried by suspension springs and supported by the truck""s wheels. Wheel suspension springs have very high spring rates which makes the ride jarring to the operator and to structures supported by the frame. To dampen the vibration and shock transmitted to the cab, suspension systems have been employed utilizing conventional suspension components such as air springs and shock absorbers to isolate the cab structure from shock and to dampen motion of the cab, respectively.
Cab suspensions have been situated close to the center line of the vehicle, and have positioned the air spring and primary damping elements of the suspension inward from the frame rails and underneath the rear sill of the cab. Due to space constraints and the desirability of enhancing damping to cab roll, prior art suspension systems have positioned shock damping devices offset from the centerline of the rear sill toward the front of the vehicle. Off centerline shock damping induces a bending moment or twist to the cab rear sill, inducing stress in the cab, particularly between the sill and sheet metal floor panels welded to the sill.
An example of such a cab suspension system is described in U.S. Pat. No. 5,109,939 to Conaway et al. In order to position suspension elements close to the vehicle center line under the cab, a rigid base member perpendicular to and breaching the gap between the frame rails is placed under the rear of the cab. A rigid impact plate is provided above the base member and parallel thereto for attachment to the cab bottom. Between the base member and the impact plate are a pair of air springs oriented for vertical compression and aligned across the direction of elongation of the vehicle. A pair of shock absorbers are located toward the front of the vehicle just forward from the air springs. The shock absorbers are aligned on one another perpendicular to the direction of elongation of the vehicle and both are inwardly canted from bottom to top and pivotally mounted to the impact plate. The central axes of the shock absorbers and the support member and impact plate generally form a trapezoid, controlling roll, but resulting in twisting forces transmitted through the shock absorbers about the cab""s rear sill.
According to the invention there is provided a vehicle having a suspended cab. The vehicle has a chassis with elongated side frame rails and cross members connected between the side frame rails. One cross member provides support for the back of a vehicle cab. A cab attachment plate is supported above the cross member and provides for attachment to a cab rear sill in the floor of the cab. A pair of air springs is disposed, latitudinally aligned between the attachment plate and the cross member, for supporting the cab. Fore and aft shock absorbers, aligned along the vehicle center line are attached between the cab attachment plate and the cross member member for damping vertical movement of the cab.
Additional effects, features and advantages will be apparent in the written description that follows.